1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy and, more particularly, to methods which allow gases produced during the formation stage of a cell, such as the formation stage of a lithium-ion cell, to be released from the casing before the cell is placed into service.
2. Prior Art
Lithium-ion rechargeable cell must be xe2x80x9cformedxe2x80x9d after assembly. Formation occurs during the first several charge-discharge cycles. During this cycling, lithium ions travel through the electrolyte from the cathode to the anode upon external charging and return to the cathode during discharging of the cell. The charge capacity exceeds the discharge capacity during the first charge-discharge cycle because not all of the lithium retained by the anode is released back to the cathode. In other words, during each cycle some of the lithium is irreversibly retained by the carbonaceous anode. This irreducible lithium causes the anode to swell. Consequently, the casing swells. Also, decomposition of the electrolyte during the formation cycles causes gases to be produced. These gases increase the pressure inside the casing and result in additional case swelling.
According to one embodiment of the present invention, a temporary one-way gas release valve is positioned on the casing, in fluid flow communication between the inside thereof and the exterior. This temporary gas release valve serves to eliminate gases that build up inside the casing during the cell""s formation stage. Once the lithium-ion cell has completed formation, the temporary gas release valve is removed and replaced with a hermetic closure. Removal of the release valve and sealing of the cell takes place in an environment in which no outside gas is capable of being introduced into the casing.
Various prior art patents describe gas release valves for electrochemical cells. These include U.S. Pat. No. 3,208,884 to Jensen and U.S. Pat. No. 3,398,026 to Andrxc3xa9. However, none of them disclosure use of the release valve only during the formation stage of a lithium-ion rechargeable cell. After the formation stage is completed, the present invention release valve is removed from the cell casing and replaced with a closure means, which hermetically seals the cell.
According to another embodiment of the present invention, lithium-ion cells are formed in a housing with a temporary opening to an inert atmosphere. After formation is completed, the cell is sealed with a hermetic closure and discharged in a normal manner.
An object of the present invention is, therefore, to reduce the swelling experienced by a lithium-ion rechargeable cell by eliminating gases which form inside the casing during the cell""s formation stage. One way to accomplish this is by the provision of a one-way gas release valve provided on the casing. Upon completion of cell formation, the temporary gas release valve is removed from the casing under conditions which prevent any outside gas from entering the electrochemical system. The opened casing is then closed with a hermetic seal and discharged in the usual manner. Another way to form the cell is by containing it in a housing having an inert environment, such as of nitrogen or argon. The cell has a vent opening to the inert environment during its formation stage. After formation is completed, the cell is hermetically sealed. Such batteries are particularly useful as the power source for an implantable medical device, including ventricular assist devices, defibrillators, pacemakers, neural stimulators, drug pumps and hearing aids.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the appended drawings.